Yea, right now, *neither* execution mode is going to give you that
granular control, unfortunately. They're both oriented towards a "run
these directives on all servers" workflow.

I'm more than halfway done with a massive rewrite of Fabric which, if
it works out, should make it a lot easier to leverage as a library.
While I may not have your needed level of atomicity available when it
first comes out, it's definitely something on my radar.